1. Field of the Invention
This invention relates to the preparation of at least bis-carbamate functional 1,3,5-triazines from amino-1,3,5-triazines and organic carbonates in the presence of a base, as well as to certain novel carbamates producible thereby.
2. Description of Related Art
Various derivatives of amino-1,3,5-triazines are described in the literature as being utilized in a wide variety of fields. An important use of certain of these derivatives, such as the methoxymethyl derivatives of melamine and guanamines, is as crosslinkers and/or reactive modifiers in curable compositions based upon active hydrogen groups-containing resins. While these methoxymethyl derivatives provide excellent results in a number of aspects, one disadvantage to their use is that, under curing conditions, formaldehyde is released as a volatile by-product. It has, therefore, been a desire of industry to find an acceptable alternative for these well-known amino-1,3,5-triazine derivatives which do not emit formaldehyde upon cure.
One such alternative which has shown great promise is the carbamate functional 1,3,5-triazines which are disclosed in commonly owned U.S. Pat. Nos. 4,939,213, 5,084,541, U.S. application Ser. No. 07/968,871 (filed Oct. 30, 1992, which is a continuation-in-part of U.S. application Ser. No. 07/793,077, filed Nov. 15, 1991 and now abandoned), and U.S. application Ser. No. 07/998,313 (filed Dec. 29, 1992), all of which are hereby incorporated by reference as if fully set forth. Specifically, the carbamate functional 1,3,5-triazines disclosed in these references have been found to be particularly useful as crosslinkers in coating compositions based upon hydroxy functional resins, with the cured coatings possessing a wide range of desirable film and resistance properties.
One hinderance to the commercial use of these carbamate functional 1,3,5-triazines has been that the known preparation methods can be somewhat cumbersome, difficult and expensive. For example, in previously mentioned U.S. Pat. Nos. 4,939,213 and 5,084,541, the 1,3,5-triazine carbamates are produced in a two-step process by first reacting an amino-1,3,5-triazine with oxalyl chloride to produce an isocyanate functional intermediate, then reacting this intermediate with an alcohol. The primary disadvantages to this process include the use of a halogenated starting material, the handling of an isocyanate functional intermediate, the production of halogenated by-products and the low ultimate yield of the desired carbamate functional product.
Further, in previously mentioned U.S. application Ser. No. 07/968,671, the carbamate functional 1,3,5-triazines are produced in a one-step process by reacting a haloaminotriazine with an acid halide. The primary disadvantages to this process include the use of halogenated starting materials and the production of halogenated by-products.
It has now been surprisingly discovered after extensive research that at least bis-carbamate functional 1,3,5-triazines can be readily produced with high selectivity and in excellent yields by the reaction of (a) at least diamino-1,3,5-triazines and (b) acyclic organic carbonates in the presence of relatively strong bases.
It should be noted that it is generically known to obtain carbamates via the reaction of amines with carbonates. It is, however, well-known to those of ordinary skill in the art that the amine functionality of amino-1,3,5-triazines (such as the amine functionality of melamines and guanamines) is not equivalent to the other types of typical amine functionality. Indeed, melamines and guanamines are among the least reactive of the "amines" and the most difficult to functionalize, and their behavior cannot normally be correlated to that of other known amines, even structurally similar amines such as pyrimidines.
For example, most "typical" amines are highly reactive with acid halides. In a publication by E. M. Smolin and L. Rapaport entitled "S-Triazines and Derivatives," Interscience Publishers Inc., New York, page 333 (1959), however, it is reported that attempts to react an acid halide with the amino group on a 1,3,5-triazine such as melamine have not been successful.
Further, attempts to functionalize amino-1,3,5-triazines often results in substitution at the nitrogen on the triazine ring. For example, it is known that the reaction of melamine with alkyl halides, such as allyl chloride, results in alkyl substitution at the nitrogen on the triazine ring resulting in isomelamine derivatives.
To date, a disclosure has not been found whereby an at least bis-carbamate functional 1,3,5-triazine has been prepared from an at least diamino-1,3,5-triazine and an organic carbonate. While several references appear to generically disclose (among other techniques) that mono-carbamate 1,3,5-triazines can be prepared by the reaction of a mono-amino 1,3,5-triazine and an organic carbonate (see U.S. Pat. Nos. 5,017,212, 4,579,583, 4,878,938, 4,480,101, and 4,804,757, and PCT Unexamined Application Ser. No.9206965), no specific example could be found in these references of any mono-carbamate 1,3,5-triazine actually having been so prepared. The only disclosure found exemplifying a mono-carbamate 1,3,5-triazine produced from a monoamino-1,3,5-triazine and an organic carbonate is in Research Disclosure, Vol. 275, p. 162 (1987).
Quite relevant to the surprising nature of the present invention is an unsuccessful attempt to prepare a 1,3,5-triazine-2,4,6-tris-carbamate from an amino triazine and ethyl chloroformate (a potential precursor component of an organic carbonate), which is reported in an article by H. Kitajima et al in Yuki Gosei Kagaku Kyokai Shi, Volume 32, Number 9, page 723-26 (1974) (Chemical Abstracts, Volume 82, Number 11:72946). Instead of producing the desired tris-carbamate, the authors reported the formation of an imide via diacylation of the amino groups. Similarly, the reaction of benzoguanamine with ethyl chloroformate produces N,N-diethoxycarbonyl benzoguanamine. Such imide functional compounds are unsuitable as crosslinking agents. Similar results are also reported in Rec. Trav. Chim., Vol. 81, pages 241-254 (1962).
In fact, it is clearly surprising in light of the state of the art that at least bis-functional carbamate derivatives can be produced with high selectivity and yield from the at least diamino-1,3,5-triazines via the present inventive process, as described in detail below.